I am trying to cout a const char *
This is how I convert an int to a string and concatenate it with the const char*
char tempTextResult[100];
const char * tempScore = std::to_string(6).c_str();
const char * tempText = "Score: ";
strcpy(tempTextResult, tempText);
strcat(tempTextResult, tempScore);
std::cout << tempTextResult;
The result when printing is: Score:
Does anyone know why the 6 is not printing?
Thanks in advance.
As the docs for c_str say, "The pointer returned may be invalidated by further calls to other member functions that modify the object." This includes the destructor.
const char * tempScore = std::to_string(6).c_str();
This makes tempScore point to a temporary string that no longer exists. You should do this:
std::string tempScore = std::to_string(6);
...
strcat(tempTextResult, tempScore.c_str());
Here, you're calling c_str on a string that continues to exist.
You have marked this post as C++.
One possible C++ approach: (not compiled, not tested)
std::string result; // empty string
{
std::stringstream ss;
ss << "Score: " // tempText literal
<< 6; // tempScore literal
// at this point, the values placed into tempTextResult
// are contained in ss
result = ss.str(); // because ss goes out of scope
}
// ss contents are gone
// ... many more lines of code
// ... now let us use that const char* captured via ss
std::cout << result.c_str() << std::endl;
// ^^^^^^^ - returns const char*
Related
void reverse(char * str){
char * end = str;
cout << "str" << str << endl;//ABCDE
cout << "end" << end << endl;//ABCDE
char tmp;
if(str){
while(*end){++end; cout << end << endl;}//ABCDE-->BCDE-->CDE-->DE-->E--> NULL
--end;//end=E
cout <<"--end" << end << endl;
while(str<end){// do swap
tmp = *str;//*str = str[0]
*str++ = *end;//*end = last ele in str[]
*end-- = tmp;
}
}
}
My input is
char test[] = "ABCDE";
cout << test << endl; //ABCDE
reverse(test);
cout << test << endl; //EDCBA
I am feeling not good about the pointer, since c++ primer book says char* pointer to the first element of an array, but when I output the pointer end, it is the content of an array not the address.
Also, reverse(test), I mean to give the address of the first element in an array to the pointer, but it turns out give the whole elements to the pointer.
A char* variable is a pointer to a char. A char[] is an array of char. Now, an array of char can be accessed through a pointer, and for char* it is commonly used for string processing (it's used although for other types, but for char it's much more common).
char test[6] = "ABCDE";
char *start = &test[0]; // will point on A
Accessing the array with the pointer can be done with pointer arithmetic:
char *end = start + 5; // equivalent to char *end = &test[5]
Now when you do:
cout << test;
or
cout << start;
It's actually calling an overload of operator<< that takes a const char*. What this operator does is that it print char starting from the pointer passed until it reaches a null char ('\0').
If you want to print the address contained in the pointer and not the string, you have to cast it to void*:
cout << static_cast<void*>(start);
std::cout is overloaded to print strings for char* .
Try:
char *test = "ABCDE";
std::cout << (void *) test << std::endl;
In C++ string literals "Hello" are const and are immutable. I wanted to make a custom string class whose strings are not const chars, so they can be changeable
Here is a snippet of code that might illustrate what I'm trying to do:
#include <iostream>
class String {
public:
char * p_start;
String(char * strSourc) // Constructor
{
p_start = strSourc;
}
};
int main()
{
String myString("Hello");
// Create object myString, send "Hello" string literal as argument
std::cout << myString.p_start << std::endl;
// Prints "Hello"
*myString.p_start = 'Y';
// Attempt to change value at first byte of myString.p_start
std::cout << myString.p_start << std::endl;
// Prints "Hello" (no change)
myString.p_start = "Yellow";
// Assigning a string literal to p_start pointer
std::cout << myString.p_start << std::endl;
// Prints Yellow, change works. I thought myString "Hello" was const chars, immutable
return 0;
}
So, I'm confused. I've looked everywhere and it says that string literals, like "Hello", are immutable, each of their char bytes are unchangeable. Though I managed to assign Yellow to the p_start pointer, changing the first letter. Though changing the single letter H to a Y through dereferencing the H pointer didn't do anything.
Any insights would help me, thanks.
I think you're confusing about pointer and pointee.
p_start = "Yellow", you're changing the value of pointer, to point to "Yellow". *p_start = 'Y', you're changing the value of pointee, the content p_start points to, not itself. As you said, "Yellow" are const chars, so the behaviour try to modify them is UB.
You can make a copy of it in the ctor, then you can modify the chars, which are managed by the class. Yes, it will be a new copy, but it won't be a waste of memory. And you have no choice if you want them to be changable.
not sure if anyone finds this helpful after so long but since I am learning myself I took your above code and made it work by copying. It now has a member variable for size and cleans up using delete when you assign a new string literal to it (const char*).
#include <iostream>
class String {
public:
char * p_start;
int m_size;
String(const char * strSourc) // Constructor
{
//The following will get the size of the parameter.
int SizeParameter=0;
while (*(strSourc+SizeParameter) != '\0')
{
SizeParameter++;
}
// size of string saved.
m_size = SizeParameter;
// allocate enough memory so we can copy strSourc
p_start = new char[SizeParameter+1];
//copy the contents strSourc
for(int i=0; i<SizeParameter+1; i++)
{
*(p_start+i) = *(strSourc+i);
}
}
//Handle change of string value.
char* AssignNewString (const char* newtext)
{
//clean
delete p_start;
int SizeParameter=0;
while (*(newtext+SizeParameter) != '\0')
{
SizeParameter++;
}
// size of string saved.
m_size = SizeParameter;
// allocate enough memory so we can copy strSourc
p_start = new char[SizeParameter+1];
//copy the contents strSourc
for(int i=0; i<SizeParameter+1; i++)
{
*(p_start+i) = *(newtext+i);
}
return p_start;
}
char* operator=(const char* newtext)
{
AssignNewString(newtext);
}
};
int main()
{
String myString("Hello");
// Create object myString, send "Hello" string literal as argument
std::cout << "string size: " << myString.m_size << std::endl;
std::cout << myString.p_start << std::endl;
// Prints "Hello"
*myString.p_start = 'Y';
// Attempt to change value at first byte of myString.p_start
std::cout << myString.p_start << std::endl;
// Prints "Hello" (no change)
myString = "yellow";
// Assigning a string literal to p_start pointer
myString = "THIS IS A LONGER STRING";
std::cout << myString.p_start << std::endl;
std::cout << "string size: " << myString.m_size << std::endl;
return 0;
}
Note I am learning myself so do let me know if I am doing something wrong. But so far, it seems to work.
I have an issue using stringstreams
When I run this code the first printf is fine but at some point it gets contaminated and prints out a shorter string.
string CJpsURI::getURIwithSipIpPort()
{
stringstream ss;
ss << "sip:" << m_ipAddr << ":" << m_portAddr;
string out = ss.str();
}
main() {
CJpsURI localIp();
localIp.setIpPort("192.168.88.1", 5060);
char *s = localIp.getURIwithSipIpPort().c_str();
printf("This is the sip port: %s", s); // this may be ok -- sip:192.168.88.1:5060
// do some stuff
printf("This is the sip port: %s", s); // wrong; -- sip:192.168.8/030/004
}
It almost appears that the *s is pointing to the out string on the stack which gets destroyed. But that should not be happening since I am returning out and not a reference to out.
But this seems to work.
string CJpsURI::getURIwithSipIpPort()
{
string out = (boost::format("sip:%1%:%2%") % m_ipAddr % m_portAddr).str();
return out;
}
main() {
CJpsURI localIp();
localIp.setIpPort("192.168.1.1", 5060);
char *s = localIp.getURIwithSipIpPort().c_str();
printf("This is the sip port: %s", s); // this may be ok
// do some stuff
printf("This is the sip port: %s", s); // this will be ok too;
}
Any ideas would be appreciated
You have two problems:
As remyabel and Galik pointed out: Your function does not return. So I would Concur with Galik that you need to adapt your function to:
string CJpsURI::getURIwithSipIpPort()
{
stringstream ss;
ss << "sip:" << m_ipAddr << ":" << m_portAddr;
return ss.str();
}
A char* points to an array of chars. Here you want the array found inside the string returned by getURIwithSipIpPort. But that memory will be released as soon as this line ends! There is nothing hanging on to it. So what you really need to do is:
string s{localIp.getURIwithSipIpPort()};
The C++ standard says:
§ 6.7/2 [..] Flowing off the end of a function is equivalent to a
return with no value; this results in undefined behavior in a
value-returning function.
The second problematic statement is:
char *s = localIp.getURIwithSipIpPort().c_str();
First, the conversion from const char* to char* is deprecated. Use const char*. Second, don't do this in the first place. If you want to prolong the lifetime of a temporary, use a const reference.
const std::string& s = localIp.getURIwithSipIpPort().c_str();
Your first function does not return anything:
string CJpsURI::getURIwithSipIpPort()
{
stringstream ss;
ss << "sip:" << m_ipAddr << ":" << m_portAddr;
string out = ss.str(); // This does not get returned!
}
Try this:
string CJpsURI::getURIwithSipIpPort()
{
stringstream ss;
ss << "sip:" << m_ipAddr << ":" << m_portAddr;
return ss.str(); // Now it returns something!
}
here the comment2 is printed perfectly. where as comment is not printed and the
program ends as soon as it that statement is executed.
Can anybody provide solution?
#include <iostream>
int main()
{
const char * comment = 0;
const char * comment2 = "hello this is not empty";
std::cout << std::endl;
std::cout << comment2 << std::endl;
std::cout << "printing 0 const char *" << std::endl;
std::cout << comment << std::endl;
std::cout << "SUCCESSFUL" << std::endl;
}
Dereferencing a null pointer is undefined behaviour, and this makes comment a null pointer:
const char * comment = 0;
If you want an empty string change to:
const char* comment = "";
Or use std::string:
std::string comment;
Assigning a pointer to 0 means assigning it to NULL. If you want the character 0, change it to string, "0" or for empty string, "".
const char * comment = "";
std::cout << comment << std::endl;
When comment is 0, we call this a segmentation fault, and is a catastrophic crash. What do you intend to happen here?
do you want const char * comment = "0"; to print 0
You can do const char * comment = ""; to represent empty string.
const char * is a pointer. When 0 is assigned to it, it becomes a null pointer because it is now a pointer to null. When you perform cout, the library tries to access the memory at that location, a process called dereferencing a pointer. This causes a crash as explained below.
From Wikipedia
Dereferencing a null pointer in C produces undefined behavior,[5] which could be catastrophic. However, most implementations[citation needed] simply halt execution of the program in question, usually with a segmentation fault.
const char * comment = 0;
is equal to
const char * comment = NULL;
if you want to print character 0, try below code:
const char * comment = "0";
As you tag c++, better use
std::string comment("0");
I have a static method that should return the next available id. Several static members in other classes call this function, so each gets a different id assigned. However, when I am printing the assigned values I dont get "id0", "id1" but just a symbol like "*". I'm using this code:
int A::i = 0; //static member
std::string A::id()
{
std::stringstream sst;
sst<< "id" << A::i;
i++;
return sst.str(); //i've tried to return a const char* too, but doesnt work either
}
//assigning id's in several classes like this:
const char* B::id = A::id().c_str();
const char* C::id = A::id().c_str();
//printing them in the main function:
std::cout << B::id << " " << C::id << "\n";
I dont understand why the code above doesnt work. When I am executing the following code the expected result "id10" is printed:
static std::string gt()
{
std::stringstream ss;
ss << "id" << 10;
return ss.str();
}
Look at
const char* C::id = A::id().c_str();
You are creating a new id string, then you take a pointer to its data (as a c-string), and inmediately after that the temporary string to which contents you are pointing gets destroyed. Simple answer: either stick to std::string or to plain int ids.